U.S. patent application number 09/782393 was filed with the patent office on 2002-08-15 for deep reactive ion etching process and microelectromechanical devices formed thereby.
Invention is credited to Christenson, John C., Rich, David Boyd.
Application Number | 20020109207 09/782393 |
Document ID | / |
Family ID | 25125910 |
Filed Date | 2002-08-15 |
United States Patent
Application |
20020109207 |
Kind Code |
A1 |
Rich, David Boyd ; et
al. |
August 15, 2002 |
Deep reactive ion etching process and microelectromechanical
devices formed thereby
Abstract
A process for forming a microelectromechanical system (MEMS)
device by a deep reactive ion etching (DRIE) process during which a
substrate overlying a cavity is etched to form trenches that breach
the cavity to delineate suspended structures. A first general
feature of the process is to define suspended structures with a
DRIE process, such that the dimensions desired for the suspended
structures are obtained. A second general feature is the proper
location of specialized features, such as stiction bumps,
vulnerable to erosion caused by the DRIE process. Yet another
general feature is to control the environment surrounding suspended
structures delineated by DRIE in order to obtain their desired
dimensions. A significant problem identified and solved by the
invention is the propensity for the DRIE process to etch certain
suspended features at different rates. In addition to etching wider
trenches more rapidly than narrower trenches, the DRIE process
erodes suspended structures more rapidly at greater distances from
anchor sites of the substrate being etched. At the masking level,
the greater propensity for backside and lateral erosion of certain
structures away from substrate anchor sites is exploited so that,
at the completion of the etch process, suspended structures have
acquired their respective desired widths.
Inventors: |
Rich, David Boyd; (Kokomo,
IN) ; Christenson, John C.; (Kokomo, IN) |
Correspondence
Address: |
JIMMY L. FUNKE
DELPHI TECHNOLOGIES, INC.
Legal Staff Mail Code A-107
P.O. Box 9005
Kokomo
IN
46904-9005
US
|
Family ID: |
25125910 |
Appl. No.: |
09/782393 |
Filed: |
February 14, 2001 |
Current U.S.
Class: |
257/586 ;
257/622; 438/690; 438/800 |
Current CPC
Class: |
G01P 15/0888 20130101;
H01L 21/67069 20130101; G01P 15/125 20130101; B81B 2201/025
20130101; B81C 1/00587 20130101; B81C 2201/0132 20130101; B81B
2203/0118 20130101; B81C 1/00404 20130101; G01P 15/0802
20130101 |
Class at
Publication: |
257/586 ;
257/622; 438/690; 438/800 |
International
Class: |
H01L 027/082; H01L
027/102; H01L 029/70; H01L 031/11; H01L 029/06; H01L 021/302; H01L
021/461; H01L 021/00 |
Claims
1. A process of forming a microelectromechanical device by a deep
reactive ion etching process during which a substrate overlying a
cavity is etched to form trenches that breach the cavity to
delineate at least two suspended structures, a first and a second
of the suspended structures having a first and a second
predetermined width, respectively, in a direction parallel to a
surface of the substrate, the first suspended structure being
farther from an anchor site of the substrate than the second
suspended structure, the process comprising the steps of: masking
first and second surface regions of the substrate corresponding to
the first and second suspended structures, respectively, in
preparation for the etching process so that other surface regions
of the substrate corresponding to the trenches remain exposed, the
first masked surface region being wider in the direction parallel
to the surface than the first predetermined width of the first
suspended structure; and then forming the first and second
suspended structures by the etching process during which the
exposed surface regions of the substrate are etched to form the
trenches that delineate the first and second suspended structures
and breach the cavity, after which the first suspended structure is
subjected to more rapid backside and lateral erosion than the
second suspended structure so that at the completion of the etching
process the first and second suspended structures have the first
and second predetermined widths, respectively.
2. The process according to claim 1, wherein the second masked
surface region has a width in the direction parallel to the surface
that is approximately equal to the second predetermined width of
the second suspended structure.
3. The process according to claim 2, wherein the first and second
predetermined widths are approximately equal.
4. The process according to claim 1, wherein the
microelectromechanical device comprises a proof mass supported
above the cavity and a rim surrounding the proof mass and the
cavity.
5. The process according to claim 4, wherein the first suspended
structure is one of a first plurality of fingers projecting from
the proof mass and each of the first plurality of fingers has a
width substantially equal to the first predetermined width, the
second suspended structure is one of a second plurality of fingers
projecting from the rim and each of the second plurality of fingers
has a width substantially equal to the second predetermined width,
and the first and second plurality of fingers are
interdigitized.
6. The process according to claim 5, further comprising a stiction
bump on each of the second plurality of fingers facing an adjacent
one of the first plurality of fingers.
7. The process according to claim 5, further comprising multiple
stiction bumps on a surface of the cavity beneath only the proof
mass.
8. The process according to claim 7, wherein the stiction bumps are
not present on the surface of the cavity beneath any of the
trenches.
9. The process according to claim 1, wherein the first and second
suspended structures are first and second portions of a single
suspended element such that the first portion of the suspended
element is farther from the anchor site of the substrate than the
second portion of the suspended element, the first and second
masked surface regions define a continuous masked surface region
corresponding to the suspended element during the masking step, the
continuous masked surface region is between two exposed surface
regions of the substrate during the masking step, and the suspended
element is between the trenches that delineate the first and second
portions of the suspended element following the forming step.
10. The process according to claim 9, wherein the continuous masked
surface region corresponding to the suspended element has a uniform
width, portions of the two exposed surface regions adjacent the
second portion of the suspended element are wider than portions of
the two exposed surface regions adjacent the first portion of the
suspended element such that each of the two exposed surface regions
are tapered, and prior to the cavity being breached during the
forming step, the portions of the two exposed surface regions
adjacent the second portion of the suspended element etch more
rapidly than the portions of the two exposed surface regions
adjacent the first portion of the suspended element.
11. The process according to claim 9, wherein the first masked
surface region corresponding to the first portion of the suspended
element is wider than the second masked surface region
corresponding to the second portion of the suspended element such
that the continuous masked surface region corresponding to the
suspended element is tapered, and each of the two exposed surface
regions have uniform widths.
12. The process according to claim 1, wherein one of the first and
second suspended structures is more fragile than the other.
13. The process according to claim 12, wherein as a result of the
masking step, a first of the exposed surface regions delineates the
more fragile of the first and second suspended structures and a
second of the exposed surface regions delineates the other of the
first and second suspended structures, the second exposed surface
region being wider than the first exposed surface region, the
trench etched in the second exposed surface region breaching the
cavity before the trench etched in the first exposed surface region
during the forming step.
14. The process according to claim 1, wherein a portion of the
substrate overlying the cavity is deflected into the cavity prior
to the trenches breaching the cavity during the forming step.
15. The process according to claim 14, further comprising multiple
stiction bumps on a surface of the cavity beneath the substrate
overlying the cavity, the stiction bumps being located on the
surface of the cavity so as not to be contacted by the portion of
the substrate deflected into the cavity.
16. A process of forming a microelectromechanical device that
includes a substrate overlying a cavity and a rim surrounding the
cavity, the process being a deep reactive ion etching process by
which the substrate is etched to form trenches that breach the
cavity to delineate multiple suspended structures, the multiple
suspended structures comprising a proof mass supported above a
floor of the cavity and separated from a central hub on the floor
by a first of the trenches first fingers cantilevered radially
outward from the proof mass and interdigitized with second fingers
cantilevered radially inward from the rim and spaced apart from the
first fingers by second trenches, and tethers suspended between and
interconnecting the proof mass and the rim with a first portion of
each tether being adjacent the proof mass and a second portion of
each tether being adjacent the rim, each of the tethers being
between a pair of third trenches, the first and second fingers and
the first and second portions of the tethers having respective
predetermined widths in a direction parallel to a surface of the
substrate, the trenches defining at least first and second gaps
having respective predetermined widths, the process comprising the
steps of: forming the cavity to have an annular shape surrounding
the central hub, an outer perimeter, and a first plurality of
stiction bumps on the floor of the cavity spaced apart from the
central hub and the perimeter of the cavity; masking surface
regions of the substrate corresponding to the proof mass, the first
and second fingers, and the tethers so that other surface regions
of the substrate corresponding to the trenches remain exposed, the
first plurality of stiction bumps being only beneath the masked
surface regions corresponding to the proof mass, the masked surface
regions corresponding to the first fingers being wider than the
predetermined width of the first fingers, the masked surface region
corresponding to at least one of the second fingers being masked to
define a second stiction bump on the at least one second finger
facing a corresponding one of the first fingers with which the at
least one second finger is interdigitized; and then micromachining
the proof mass, the first and second fingers, and the tethers by
the deep reactive ion etching process so that the exposed surface
regions corresponding to the trenches are etched and the cavity is
first breached by one of the trenches etched through one of the
exposed surface areas away from the first and second fingers, and
after etching the second trenches that delineate the first and
second fingers the first fingers are subject to backside and
lateral erosion that undercuts the masked surface regions
corresponding to the first fingers so that at the completion of the
etching process the first and second fingers, the first and second
portions of the tethers, and the first and second gaps have their
respective predetermined widths; wherein the first plurality of
stiction bumps are beneath only the proof mass, and the second
stiction bump is present on the at least one second finger and
faces the corresponding one of the first fingers with which the at
least one second finger is interdigitized.
17. The process according to claim 16, wherein the masked surface
regions corresponding to the second fingers have widths
approximately equal to the predetermined widths of the second
fingers.
18. The process according to claim 16, wherein the predetermined
widths of the first and second fingers are approximately equal.
19. The process according to claim 16, wherein each of the masked
surface regions corresponding to the second fingers is masked to
define a stiction bump, so that stiction bumps are formed on each
second finger and face the first fingers with which the second
fingers are interdigitized.
20. The process according to claim 16, wherein each of the masked
surface regions corresponding to the tethers has a uniform width,
portions of the exposed surface regions adjacent the second portion
of each of the tethers are wider than portions of the exposed
surface regions adjacent the first portion of each of the tethers
such that each of the exposed surface regions on either side of
each tether is tapered, and prior to the cavity being breached
during the micromachining step, the portions of the exposed surface
regions adjacent the second portions of each of the tethers etch
more rapidly than the portions of the exposed surface regions
adjacent the first portions of each of the tethers.
21. The process according to claim 16, wherein the masked surface
regions corresponding to the first portions of the tethers adjacent
the proof mass are wider than the predetermined width of the first
portions and wider than the masked surface regions corresponding to
the second portions of the tethers adjacent the rim, and the
exposed surface regions on either side of the tethers have uniform
widths.
22. The process according to claim 21, wherein the masked surface
regions corresponding to the second portions of the tethers
adjacent the rim are approximately equal to the predetermined
widths of the second portions of the tethers.
23. The process according to claim 16, wherein the cavity is first
breached by the first trench that separates the proof mass from the
central hub.
24. A microelectromechanical device comprising: a substrate having
a cavity, a floor of the cavity, and a rim surrounding the cavity;
a proof mass supported within the cavity so as to have an axis of
rotation perpendicular to the substrate; first fingers cantilevered
radially outward from the proof mass toward the rim; second fingers
cantilevered radially inward from the rim toward the proof mass and
interdigitized with the first fingers; and tethers interconnecting
the proof mass and the rim; wherein the microelectromechanical
device further comprises at least one stiction bump located on the
floor of the cavity beneath the proof mass, and a stiction bump on
at least one of the second fingers facing a corresponding one of
the first fingers.
25. The microelectromechanical device according to claim 24,
wherein the first and second fingers have approximately equal
widths in a direction parallel to a surface of the substrate.
26. The microelectromechanical device according to claim 24,
wherein each of the tethers has a substantially constant width in a
direction parallel to a surface of the substrate along a length of
the tether between the proof mass and the rim.
27. The microelectromechanical device according to claim 24,
wherein a stiction bump is present on each of the second fingers
and the stiction bumps face the first fingers with which the second
fingers are interdigitized.
28. The microelectromechanical device according to claim 24,
wherein a plurality of stiction bumps are present on the floor of
the cavity beneath only the proof mass.
29. The microelectromechanical device according to claim 24,
wherein the proof mass surrounds a hub on the floor of the cavity
and has a peripheral region adjacent the first fingers, and wherein
a first of the plurality of stiction bumps on the floor of the
cavity surround the hub, a second of the plurality of stiction
bumps on the floor of the cavity are directly beneath the
peripheral region of the proof mass, and an annular-shaped central
region of the cavity is free of stiction bumps.
Description
TECHNICAL FIELD
[0001] The present invention generally relates to micromachined
devices, and particularly microelectromechanical system (MEMS)
devices formed by etching processes. More particularly, this
invention relates to a micromachining process and design elements
for a MEMS device using a deep reactive ion etching (DRIE) process
to improve yields and device reliability.
BACKGROUND OF THE INVENTION
[0002] A wide variety of MEMS devices are known, including
accelerometers, rate sensors, actuators, motors, microfluidic
mixing devices, springs for optical-moving mirrors, etc. As an
example, rotational accelerometers that employ MEMS devices are
widely used in computer disk drive read/write heads to compensate
for the effects of vibration and shock. Other applications for
rotational accelerometers that use MEMS devices include VCR cameras
and aerospace and automotive safety control systems and
navigational systems. Rotational rate sensors and accelerometers
have been developed whose MEMS devices are fabricated in a
semiconductor chip. Notable MEMS devices that employ a proof mass
for sensing rotational rate or acceleration include a plated metal
sensing ring disclosed in U.S. Pat. No. 5,450,751 to Putty et al.,
and an electrically-conductive, micromachined silicon sensing ring
disclosed in U.S. Pat. No. 5,547,093 to Sparks, both of which are
assigned to the assignee of this invention. Sparks' sensing ring is
formed by etching a chip formed of a single-crystal silicon wafer
or a polysilicon film on a silicon or glass handle wafer. A sensor
disclosed in U.S. Pat. No. 5,872,313 to Zarabadi et al., also
assigned to the assignee of the present invention, employs a
sensing ring and electrodes with interdigitized members. The
positions of the interdigitized members relative to each other
enable at least partial cancellation of the effect of differential
thermal expansion of the ring and electrodes, reducing the
sensitivity to temperature variations in the operating environment
of the sensor. Each of the above sensors operates on the basis of
capacitively sensing movement of their rings. The sensing rings are
supported by a central hub or pedestal. Surrounding the rings are
drive electrodes that drive the rings into resonance, while sensing
electrodes that also surround the rings serve to capacitively sense
the proximity of the ring (or nodes on the ring) which varies due
to Coriolis forces that occur when the resonating ring is subjected
to rotary motion.
[0003] Another notable MEMS device that employs a silicon proof
mass for sensing rotational acceleration is disclosed in U.S.
Patent application Serial No. [Attorney Docket No. H-203587] to
Rich, incorporated herein by reference. Rich discloses a
disk-shaped proof mass supported above a cavity formed in a
substrate. Instead of being centrally supported by a pedestal,
Rich's proof mass is suspended from its perimeter with tethers
anchored to the substrate rim surrounding the proof mass. The
tethers allow the proof mass to rotate about an axis perpendicular
to the plane containing the proof mass and tethers. Fingers extend
radially outward from the proof mass and are interdigitized with
fingers extending radially inward from the substrate rim. The
cantilevered fingers of the proof mass and rim are capacitively
coupled to produce an output signal that varies as a function of
the distances between adjacent paired fingers, which in turn varies
with the angular position of the proof mass as it rotates about its
axis of rotation in response to a rotational acceleration.
[0004] Sensors of the type described above are capable of extremely
precise measurements, and are therefore desirable for use in a wide
variety of applications. However, the intricate proof masses and
associated sensing structures required for such sensors must be
precisely formed in order to ensure the proper operation of the
sensor. For example, Rich's device requires a sufficient gap
between paired interdigitized fingers to prevent stiction and
shorting, yet paired fingers must also be sufficiently close to
maximize the capacitive output signal of the sensor. Rich employs
stiction bumps formed on the proof mass fingers to inhibit stiction
between closely-spaced fingers. Increasing the area of the fingers
to achieve greater capacitive coupling would result in increased
capacitive output for a given finger gap. However, traditional
etching techniques have not generally been well suited for
massproducing silicon micromachines with high aspect ratios
necessary to etch closely-spaced fingers in a relatively thick
substrate. For example, with conventional etching techniques it is
difficult to achieve a 10:1 aspect ratio capable of forming
interdigitized fingers spaced three micrometers apart in a silicon
substrate that is thirty micrometers thick. In addition to
operational considerations, there is a continuing emphasis for
motion sensors that are lower in cost, which is strongly impacted
by process yield, yet exhibit high reliability and performance
capability. Consequently, improvements in the processing of MEMS
devices for sensing and other applications are highly desirable.
Deep reactive ion etching (DRIE) is a process known as being
capable of performing deep, high aspect ratio anisotropic etches of
silicon and polysilicon, and is therefore desirable for producing
all-silicon MEMS of the type taught by Rich. However, DRIE is a
young technology practiced largely for research and development.
Accordingly, to take advantage of the unique capabilities of the
DRIE process, its etch idiosyncrasies must be determined and
reconciled to render it suitable for mass production.
SUMMARY OF THE INVENTION
[0005] The present invention provides a process and design elements
for a microelectromechanical system (MEMS) device by a deep
reactive ion etching (DRIE) process during which a substrate
overlying a cavity is etched to form trenches that breach the
cavity to delineate suspended structures. The invention is
particularly useful in the fabrication with a DRIE process of
semiconductor MEMS devices used to sense motion or acceleration,
which typically include a proof mass suspended above a cavity so as
to have an axis of rotation perpendicular to the plane of the proof
mass, as taught by Rich, Sparks and Zarabadi et al. While the
invention will be illustrated in reference to a MEMS device with a
proof mass, the invention is applicable to essentially any
suspended structure that can be fabricated by forming a trench in a
substrate overlying a cavity.
[0006] According to the invention, in addition to a relatively
large member such as a proof mass, MEMS devices also may include
additional and smaller structures that are suspended above the same
cavity, such as the tethers and cantilevered fingers of Rich. A
first general feature of the invention is the ability to define
suspended structures with a DRIE process, such that the dimensions
desired for the suspended structures are obtained. A second general
feature of the invention is the ability to define specialized
features, such as stiction bumps that, if delineated by DRIE, must
be properly located between suspended structures in order to be
effective in improving the reliability of the MEMS device. Yet
another general feature of the invention is the control of the
environment surrounding suspended structures delineated by DRIE in
order to obtained their desired dimensions.
[0007] A significant problem identified and solved by the present
invention is the propensity for the DRIE process to etch suspended
features at different rates. DRIE has been determined to etch wide
trenches more rapidly than narrower trenches. According to the
invention, DRIE etches or, more accurately, erodes suspended
structures more rapidly at greater distances from anchor sites of
the substrate being etched, which occurs when a suspended structure
becomes isolated from the bulk substrate when the trench(s) that
delineates the structure breaches the cavity. (As used herein, an
anchor site is a location on the bulk of the substrate from which
the suspended structure is ultimately supported from the bulk of
the substrate.) Consequently, though two suspended structures are
separated by a gap of constant width, DRIE processes have been
determined to more rapidly erode the suspended structure located
farther from an anchor site. Using Rich's MEMS device as an
example, once the proof mass is separated from the bulk of the
substrate using a DRIE process. the proof mass fingers etch more
rapidly than the rim fingers because the rim fingers are anchored
(cantilevered) directly from the rim of the bulk substrate
surrounding the proof mass, while the proof mass fingers are
ultimately anchored to the rim of the bulk substrate through the
tethers that suspend the proof mass from the same rim of the bulk
substrate. A consequence of this more rapid etch is backside
erosion and lateral thinning of the proof mass fingers.
[0008] In view of the above, in order to DRIE etch a substrate
above a cavity to form suspended structures above the cavity, in
which a first of the suspended structures is farther from the
substrate anchor site than the second suspended structure, the
present invention exploits the greater propensity for backside and
lateral erosion of certain structures farther from substrate anchor
sites so that, at the completion of the etch process, all suspended
structures have acquired their respective desired widths. In this
example, first and second surface regions of the substrate
corresponding to the first and second suspended structures are
masked in preparation for the DRIE etching process, leaving exposed
those surface regions of the substrate corresponding to the
trenches that will surround and delineate the suspended structures.
The first masked surface region is intentionally wider than the
desired width of the first suspended structure, thus resulting in
the adjacent exposed surface region being narrower than the width
desired for the trench that will delineate the first suspended
structure. The first and second suspended structures are then
concurrently DRIE etched. According to the invention, as a result
of the first suspended structure being a greater distance from the
anchor site than the second suspended structure, the first
suspended structure is subject to backside and lateral erosion
after the cavity is breached, causing the first structure to narrow
and eventually acquire its desired lateral width during completion
of the etch. As a result, the first masked surface region is
intentionally undercut so that, at the completion of the etch
process, the first and second suspended structures have acquired
their respective desired widths.
[0009] The tendency for DRIE to etch wider trenches more rapidly
than narrower trenches, a phenomenon which may be termed "etch
lag," is also detrimental to the formation of suspended features
for the same reasons explained above. An example is where first and
second suspended structures are to be DRIE etched in a substrate
over a cavity, in which the first structure is delineated by a
wider trench than the second structure. Because the wider trenches
of the first structure etch more rapidly during DRIE etching, the
wider trenches breach the underlying cavity before the narrower
trenches of the second structure, leading to backside erosion and
lateral narrowing as explained previously. This phenomenon would be
compensated for by masking the substrate so that the masked surface
region corresponding to the first structure is intentionally wider
than desired, resulting in the adjacent exposed surface regions
being narrower than the width desired for the trench that will
delineate the first structure, yet wider than the exposed surface
regions of the substrate corresponding to the trench that will
surround and delineate the second structure. The trench formed in
the wider exposed surface regions of the first structure breaches
the underlying cavity before the narrower trench of the second
structure, with the resulting backside and lateral erosion of the
first structure causing the first structure to narrow and
eventually substantially acquire its desired width during
completion of the etch of the narrower trench surrounding the
second structure. However, this scenario is complicated by the
findings of the present invention that backside and lateral erosion
occur more rapidly with those suspended structures located farther
from an anchor site. This invention provides two approaches for
addressing this problem. A first is to taper the width of the mask
for a suspended structure while maintaining a constant gap width
for the exposed surface area in which the trench will be etched to
delineate the structure. The mask is tapered to be wider with
increasing distance from the anchor site, so that as the width of
the mask increases, backside and lateral erosion is correspondingly
more rapid to eventually produce a substantially uniform width for
the structure. Alternatively, the width of the mask for a suspended
structure is maintained constant while tapering the gap width for
the exposed surface area in which the trench will be etched to
delineate the structure. The gap is tapered to be wider with
decreasing distance from the anchor site, so that backside and
lateral erosion of the structure that occurs more rapidly with
increasing distance from the anchor site is balanced by the more
rapid etch rate associated with the increasingly wider gap near the
anchor site. As a result, a substantially uniform width for the
structure can again be obtained.
[0010] The teachings of this invention concerning the relationship
between distance to an anchor site and backside and lateral erosion
is also pertinent to other aspects of DRIE etching a MEMS device.
As previously noted, a feature of the invention is the ability to
properly define specialized elements, such as stiction bumps.
According to the invention, stiction bumps must be defined in
regions of the substrate away from those areas in which accelerated
backside and lateral erosion will occur. Also a feature of this
invention is maintaining a proper environment surrounding suspended
structures, such as by eliminating unnecessary variations in trench
width. An important example is avoiding intersecting trenches that
would create a localized wider gap prone to more rapid etching and
subsequent backside and lateral erosion, resulting in vertical
notches at the intersections.
[0011] In view of the above, it can be seen that the present
invention provides a DRIE etching process by which suspended
structures of desired widths can be more precisely formed. As a
result, the present invention is able to take advantage of the deep
etching capability of the DRIE process, while compensating for etch
idiosyncrasies that would otherwise adversely affect the structural
integrity and durability of a MEMS device, so as to improve yields
and device reliability.
[0012] Other objects and advantages of this invention will be
better appreciated from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a plan view of a MEMS device in accordance with a
preferred embodiment of this invention.
[0014] FIGS. 2 and 3 are plan and cross-sectional views,
respectively, of a portion of the MEMS device of FIG. 1.
[0015] FIG. 4 is a detailed plan view of several interdigitized
proof mass and rim fingers of the MEMS device of FIG. 1.
[0016] FIG. 5 is a plan view of the floor of the cavity in which
the MEMS device of FIG. 1 is located.
[0017] FIGS. 6 and 7 are plan views showing portions of an etch
mask used in the fabrication of the fingers and tethers,
respectively, of the MEMS device of FIG. 1.
[0018] FIG. 8 is a plan view showing a portion of an alternative
etch mask used in the fabrication of the tethers of the MEMS device
of FIG. 1.
[0019] FIGS. 9 and 10 schematically illustrate a DRIE etch
idiosyncracy identified with the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] FIGS. 1 through 5 represent a MEMS device 10 fabricated with
a DRIE process in accordance with the present invention, with
masking steps of the DRIE process being represented in FIGS. 6
through 8. The device 10 is represented as a rotational
accelerometer of the type disclosed by Rich, which is incorporated
herein by reference. However, those skilled in the art will
appreciate that the device 10 could be employed and modified for a
variety of applications, including the rate sensors and
accelerometers taught by Putty et al., Sparks and Zarabadi et
al.
[0021] As illustrated, the device 10 includes a proof mass 14
formed in a sensing die 12. The die 12 is shown as including a
semiconductor layer 12b on a substrate 12a (FIG. 3). A preferred
material for the semiconductor layer 12b is epitaxial silicon and a
preferred material for the substrate 12a is single-crystal silicon,
though it is foreseeable that other materials could be used. For
example, the substrate 12b could be formed of quartz, glass or any
other advantageous substrate to which the semiconductor layer 12b
could be bonded. In a preferred embodiment, a wafer containing the
die 12 is processed by a known bond-etchback process, by which the
substrate 12a is etched to form a cavity 20 and then oxidized to
form a bond oxide layer (not shown) on its surface including the
cavity 20, and the semiconductor layer 12b is epitaxially grown on
a second wafer (not shown) and then bonded to the bond oxide layer
of the substrate 12a. The second wafer is then selectively removed
to leave only the epitaxial layer 12b on the substrate 12a and
overlying the cavity 20, as shown in FIG. 3. While a bond-etchback
process is preferred, it is foreseeable that other techniques could
be used to produce the die 12 and enclosed cavity 20 of FIG. 3.
[0022] As also seen in FIGS. 1 through 3, the proof mass 14 is
defined in the semiconductor layer 12b so as to be suspended above
the floor 18 of the cavity 20 between a central hub 16 and a rim 22
formed by the semiconductor layer 12b. The proof mass 14 is
attached to the bulk of the die 12 (through the semiconductor layer
12b) with four equiangularly-spaced tethers 30, and is completely
separated from the hub 16 by a trench 17, whose width may be on the
order of about seven micrometers. The tethers 30 provide that the
primary and desired translational mode of the proof mass 14 is
rotation within the plane of the proof mass 14 about the hub
16.
[0023] As seen in FIGS. 1 and 2, electrode fingers 24 radially
extend outward from the proof mass 14, and are interdigitated with
electrode fingers 26 that radially extend inward from the rim 22.
The fingers 24 and 26 are separated by trenches 28 and roughly
equiangularly spaced around the perimeter of the proof mass 14. In
the preferred embodiment, the trenches 28 are of alternating
greater and lesser widths, as is most readily apparent from FIG. 4,
though it is foreseeable that a constant trench width could be
used. Each of the narrower trenches 28 defines a capacitive gap
between a pair of smooth capacitor plates defined by the pair of
fingers 24 and 26 it separates. In contrast, the wider trenches 28
(which may be, for example, twice the width of the narrower
trenches 28) provide air gaps that separate each pair of
capacitively-coupled fingers 24 and 26 from adjacent paired fingers
24 and 26. These air gaps are effectively parasitic gaps, in that
they do not positively contribute to device performance as do the
capacitive gaps. The capacitor plates provided by the fingers 24
and 26 are preferably large relative to the width of the narrower
trench 28 therebetween, which preferably has a uniform width of,
for example, about three micrometers. When a voltage potential is
present between pairs of capacitively-coupled fingers 24 and 26,
the rim fingers 26 capacitively sense the proximity of the proof
mass fingers 24, which will vary when the proof mass 14 is
subjected to rotary motion. The large number of interdigitated
fingers 24 and 26 of the device 10 produce a capacitive signal that
is sufficiently large to measure and manipulate.
[0024] The operational requirements of the device 10 and its
conditioning circuitry (not shown) will be appreciated by those
skilled in the art, especially in reference to Rich, and therefore
will not be discussed in any detail here. It is sufficient to say
that the performance of the device 10 is generally enhanced by
increasing the number of pairs of fingers 24 and 26, and improving
the uniformity of the capacitive gaps (narrow trenches 28) while
also minimizing the widths of the gaps. Other configurations for
the device 10 are foreseeable, depending on the intended
application and operating natural mode of the device.
[0025] As also shown in FIG. 1, each of the four tethers 30 extends
from the interior of the proof mass 14, being separated from the
proof mass 14 by trenches 32. The trenches 32 are typically on the
order of about seven micrometers in width, similar to the trench 17
separating the proof mass 14 from the hub 16. The hub and tether
trenches 17 and 32, respectively, can be termed structural gaps (as
opposed to the capacitive and parasitic gaps formed by the finger
trenches 28) in that they are inactive to device signal. However,
the tether trenches 32 are important to device performance in that
they affect the compliance and response of the device 10 to some
stimulus. The opposite end of each tether 30 is anchored to a
portion 31 of the semiconductor layer 12b, thereby supporting the
proof mass 14 and, to a lesser extent, physically limiting rotation
of the proof mass 14 relative to the rim 22. Because the tethers 30
provide the structural support for the proof mass 14, they are
required to have specified widths (as measured in the plane of the
proof mass 14) and thicknesses (as measured in the direction
perpendicular to the plane of the proof mass 14) to achieve proper
rotational compliance. The tethers 30 should also be free of
irregularities, such as notches and other surface flaws that would
weaken the tether 30, increase their compliance, and provide
nucleation sites for cracks.
[0026] Stiction between the fingers 24 and 26 may still occur in
view of the very narrow trenches 28 separating them. In Rich,
stiction bumps were formed on the proof mass fingers, so as to face
the adjacent rim fingers. In the event the proof mass rotates
sufficiently to bring one or more of the proof mass fingers in
contact with their adjacent rim fingers, stiction bumps prevent
stiction, in which the fingers would permanently stick together as
a result of electrostatic forces. However, in an investigation
leading to the present invention, stiction bumps formed by DRIE on
proof mass fingers in accordance with Rich were found to be
ineffective in preventing stiction in the event of an extraordinary
rotational translational stimulus. During the investigation, a
second and unexpected source of stiction was determined to be an
undesirable translational mode of the proof mass 14 in the
Z-direction, i.e., perpendicular to the plane of the proof mass 14.
The proof mass 14, which is relatively large compared to the gap
separating it from the floor 18 of the cavity 20, can permanently
stick to the cavity floor 18 if a requisite condition is met to
cause Z-direction translation, such as a large static charge
build-up on either the cavity floor 18 or the proof mass 14 during
the DRIE etch, or water that wicks under the proof mass 14 and
evaporates, pulling the proof mass 14 down into contact with the
floor 18. The investigation leading to this invention resulted in
solutions to both of the stiction problems. For reasons to be more
fully explained below, the device 10 of this invention is
preferably fabricated to have stiction bumps 34 formed only on the
rim fingers 26, as depicted in FIGS. 3 and 4, and stiction bumps 36
formed at certain locations on the floor 18 of the cavity 20
directly beneath the proof mass 14, as shown in FIGS. 3 and 5.
[0027] The investigation was directed to the use of DRIE processing
to form the hub trench 17, finger trenches 28 and tether trenches
32 that delineate the proof mass 14 fingers 24 and 26 and tethers
30 of the device 10 shown in FIG. 1. While having different widths,
the structural trenches (hub trench 17 and tether trenches 32), the
capacitive gaps (the narrower portions of the finger trenches 28),
and the parasitic gaps (the wider portions of the finger trenches
28) preferably have a constant width along their lengths, though it
is foreseeable that trenches with variable widths could be used. In
the course of the investigation, it was determined that DRIE
consistently caused certain suspended features to etch more rapidly
than others. Specifically, the hub and tether trenches 17 and 32
(which are wider than the finger trenches 28) consistently etched
more rapidly than the finger trenches 28. Conversely, the
capacitive gaps formed by the narrower finger trenches 28 (which
are narrower than the tether trenches 32 and the parasitic gaps
formed by the wider finger trenches 28) were consistently the
slowest to etch. Consequently, as a result of etch lag associated
with DRIE, the hub trench 17 breached the cavity 20 beneath the
proof mass 14 first, followed by the tether trenches 32 and the
parasitic gaps formed by the wider finger trenches 28, and finally
the capacitive gaps formed by the narrower finger trenches 28.
However, etch lag could not account for all etch idiosyncracies
observed with the device 10.
[0028] From FIG. 1, it can be appreciated that the suspended
structures of the device 10 are not equidistant from the bulk of
the die 12, the bulk being in reference to those portions of the
die 12 other than the proof mass 14, fingers 24 and 26, and tethers
30 micromachined from the die 12. For example, the proof mass
fingers 24 are farther from the bulk of the die 12 than the rim
fingers 26, since the rim fingers 26 are anchored (cantilevered)
directly from the rim 22, which is an integral portion of the bulk
of the die 12, while the proof mass fingers 24 are ultimately
anchored to the rim 22 through the proof mass 14 and the tethers 30
that suspend the proof mass 14 from the rim 22. Surprisingly, from
the investigation it was also determined that the DRIE process more
rapidly etched suspended structures as their distances from their
anchor site (wafer bulk) increased. While not wishing to be held to
any particular theory, it is believed that this phenomenon was
caused in part by heat transfer and the highly charged environment
of the DRIE process. As the proof mass 14 becomes increasingly
isolated from the remainder of the semiconductor layer 12b as a
result of the trenches 17, 28 and 32 breaching the cavity 20, heat
transfer from the proof mass 14 to the bulk of the die 12
decreases, resulting in a temperature increase of the proof mass 14
and proof mass fingers 24 that may increase the etching rate.
Similarly, the opportunity for charge build-up in the MEMS device
10 shown in the Figures is great, with static build-up resulting in
uneven charge levels in different active regions within the die 12.
In particular, a charge build-up is likely to occur in the proof
mass 14 and its fingers 24 and tethers 30, as compared to the rim
fingers 26 and the surrounding substrate (including the rim 22),
particularly as the proof mass 14 becomes increasingly free from
the remainder of the semiconductor layer 12b as the trenches 17, 28
and 32 are completed. It was shown that once one of the more
rapidly etched trenches (e.g., the hub and tether trenches 17 and
32) breaches the cavity 20, the anisotropic nature of DRIE etching
may cause highly directional and highly energetic physical etchant
species to be reflected by the floor 18 of the cavity 20 onto the
undersides of the immediately adjacent suspended structures
(portions of the proof mass 14, fingers 24 and 26, and tethers 30),
causing both backside erosion of these structures and unintentional
lateral thinning.
[0029] The above-described phenomenon is represented in FIGS. 9 and
10, which show a masked substrate 50 over a cavity 52. In FIG. 9, a
relatively wider trench 54 (such as a tether trench 32 or a
parasitic gap 28 between fingers 24 and 26) has breached the cavity
52 before a relatively narrower trench 56 (such as a capacitive gap
between fingers 24 and 26). A first portion 58 of the substrate 50
(similar to the proof mass 14 of the device 10) is separated from
the bulk of the substrate 50 (though still suspended above the
cavity 52 and from the bulk of the substrate 50 by some suitable
structure not shown in FIGS. 9 and 10). Second and third portions
60 and 62 of the substrate 50 (similar to the proof mass and rim
fingers 24 and 26 of the device 10) remain attached to the bulk of
the substrate 50 (and are therefore closer to an anchor site on the
substrate 50 than the first portion 58). FIG. 9 also shows highly
directional and highly energetic physical etchant species 64 being
reflected by the floor of the cavity 52 onto the undersides and
sidewalls of the first and second portions 58 and 60. FIG. 10
represents the substrate 50 of FIG. 9 immediately after the
narrower trench 56 has breached the cavity 52. In FIG. 10, the
walls of the first and second portions 58 and 60 of the substrate
50 delineated by the wider trench 54 are both shown as having been
subjected to backside and lateral erosion, but the first portion 58
exhibits greater erosion and lateral thinning as a result of being
farther from the bulk of the substrate 50 than the second portion
60. In FIG. 10, the walls of the first and second portions 58 and
60 of the substrate 50 delineated by the wider trench 54 are both
shown as having been subjected to backside and lateral erosion as a
result of being adjacent the wider trench 54, which breached the
cavity 52 while the narrower trench 56 was being etched to
completion. However, the first portion 58 is shown as having
sustained greater backside erosion and lateral thinning than the
second portion 60 as a result of being subjected to more rapid
etching after breaching of the wider trench 54, since the first
portion 58 is farther from the bulk of the substrate 50 than the
second portion 60 and therefore experiences greater heating and/or
charging during the DRIE process.
[0030] In accordance with the above etch phenomenon, the wider hub
and tether trenches 17 and 32 (especially the portions of the
tether trenches 32 nearer the proof mass 14) and the wider portions
of the trenches 28 that define the parasitic gaps between fingers
24 and 26 were found to etch at a faster rate than the remaining
portions of these trenches 28 and 32, and therefore breached the
cavity 20 first. As etching progressed, erosion on the backside of
the suspended structures occurred, causing thinning of the fingers
24 and 26 and tethers 30 in the z axis (perpendicular to the plane
of the proof mass 14) and thinning of the fingers 24 and 26 and
tethers 30 in the x-y axis (in the plane of the proof mass 14).
However, those suspended structures farther from an anchor site to
the bulk of the die 12 (e.g., the proof mass fingers 24 and the
portions of the tethers 30 farthest from the substrate rim 22) were
observed to be more susceptible to backside and lateral erosion
than those suspended structures nearer an anchor site (e.g., the
rim fingers 26 and the portions of the tethers 30 nearest the
substrate rim 22). The overall effect was that the proof mass
fingers 24 and portions of the tethers 30 farthest from the rim 22
were significantly narrower than desired or acceptable. In
addition, any stiction bumps placed on the proof mass fingers 24
were eroded by excessive etching to the point that they were
completely removed, or at least their effectiveness was drastically
reduced. In addition, notches and other surface flaws were observed
during the investigation. Notching was particularly seen near the
distal ends of the fingers 24 and 26 and tethers 30 due to
energetic etch species reflection from the angled walls of the
cavity 20. Additionally, vertical notches were noted on the
sidewalls of suspended structures (fingers 24 and 26, tethers 30,
etc.) in locations where the trenches delineating the structures
(e.g., trenches 28 or 32) were intersected by a second trench. All
of the etch idiosyncrasies described above are believed to be
associated to some degree with essentially all DRIE processes in
which a suspended structure is delineated by a trench that breaches
an underlying cavity.
[0031] The present invention addresses the above defects at the
masking level by the manner in which those features of the device
10 prone to DRIE overetching are masked. In a preferred process for
fabricating the device 10 by DRIE, a suitable etch is first
performed to form the cavity 20 in the surface of the substrate 12a
(in the form of a wafer of the desired material). A suitable
technique is a wet etch of a type known in the art. Following
oxidation of the substrate 12a, the semiconductor layer 12b
(previously grown on a second wafer) is then bonded to the
substrate 12a, with the result that the cavity 20 in the substrate
12a is enclosed by the semiconductor layer 12b. Following selective
removal of the second wafer, the remaining substrate 12a and
semiconductor layer 12b yield the die 12. The surface of the die 12
is then processed in a manner well known in the art to form layers
of the MEMS device 10, after which the surface is masked to protect
surface regions of the die 12 corresponding to the proof mass 14,
the fingers 24 and 26, the tethers 30 and the surrounding rim 22. A
suitable DRIE process for use with this invention employs an
Alcatel 601 DRIE machine and a pulsed gas process in accordance
with U.S. Pat. No. 6,127,273 to Laermer et al. Another suitable
process employs an Alcatel 602 DRIE machine operated at a cryogenic
temperature in accordance with Research Disclosure No. 42271, dated
June 1999.
[0032] FIG. 6 is a detailed plan view of masking in what will be
the interdigitized finger region of the device 10, with masks 40
and 42 patterned for etching the fingers 24 and 26, respectively
(the fingers 24 and 26 and their after-etch widths are indicated in
phantom). Regions of the semiconductor layer 12b remaining exposed
between the masks 40 and 42 are also visible in FIG. 6. Notably,
while the width of the mask 42 corresponds to the width of the rim
finger 26 that will be defined by the mask 42, the mask 40 is
significantly wider than the width of the proof mass finger 24 that
it will define. As a result, the region of the semiconductor layer
12b that will be exposed by the masks 40 and 42 to the DRIE process
to form one of the trenches 28 is narrower than the desired trench
28. According to the invention, once the cavity 20 is breached, the
region of the semiconductor layer 12b corresponding to the proof
mass finger 24 beneath the mask 40 will etch more rapidly than the
region of the semiconductor layer 12b corresponding to the rim
finger 26 beneath the mask 42, causing greater undercutting beneath
the mask 40 to the extent that the desired width of the proof mass
finger 24 (shown in phantom) will be obtained. The ratio of the
width of the mask 40 relative to the desired width for the finger
24 will vary depending on various DRIE parameters, the desired
width of the trench 28, the width of the trench 28 as compared with
the widths of other trenches defining the structure, and the
thickness of the finger 24. For a proof mass finger 24 having a
desired width of about seven to eight micrometers a thickness
(based on the thickness of the semiconductor layer 12b) of about
thirty micrometers, separated from its capacitively-coupled rim
finger 26 by a trench 28 having a width of about three micrometers,
having structural trench widths of about seven micrometers, and
parasitic trench widths of about six micrometers, the mask 40
preferably extends beyond each desired edge of the proof mass
finger 24 by about 0.5 micrometers.
[0033] In reference to FIGS. 7 and 8, if the width of a mask 44 on
the semiconductor layer 12b over what will be a tether 30 (the
tether 30 and its after-etch width are indicated in phantom) were
uniform along the entire length of the tether 30, the tether 30
would be significantly narrower adjacent the proof mass 14 than
where the tether 30 attaches to the rim 22, attributable to the
greater rate of backside and lateral erosion of the tether 30 with
distances farther from the anchor site (rim 22) of the tether 30.
To counter these effects, the DRIE process of the present invention
entails masking the semiconductor layer 12b to account for the
nonuniform etching along the length of the tether 30.
[0034] In FIG. 7, the width of the mask 44 for the tether 30 is
maintained constant while tapering the gap width of the surface
area 46 exposed by the mask 44 and through which the trenches 32
will be etched to delineate the tether 30. The gap width of the
exposed surface area 46 is tapered to be wider with decreasing
distance from the rim 22 (the anchor site for the tether 30), so
that backside and lateral erosion of the tether 30 that occurs more
rapidly with increasing distance from the rim 22 is balanced by the
more rapid etch rate associated with the increasingly wider gap
near the rim 22. As a result, a substantially uniform width for the
tether 30 can be obtained. The appropriate ratio of the width of
the exposed surface area 46 relative to the desired width of the
trenches 32 will vary.
[0035] In contrast, the mask 44 for the tether 30 is shown in FIG.
8 as being tapered to increase in width toward the proof mass 14,
with the width nearest the proof mass 14 (i.e., farthest from the
tether's anchor site at the rim 22) being wider than the width
desired for the tether 30, while the width of the mask 44 adjacent
the rim 22 (i.e., nearest the rim anchor site) can be patterned to
have approximately the width desired for the tether 30. A constant
gap width is maintained for the surface area exposed by the mask 44
through which the trenches 32 will be etched to delineate the
tether 30. As the distance from the rim 22 increases, backside and
lateral erosion is correspondingly more rapid to eventually produce
a substantially uniform width for the tether 30. The appropriate
ratio of the width of the mask 44 relative to the desired width of
the tether 30 will vary, as will the amount of taper in the tether
30 itself along its length.
[0036] Significantly, the device 10 represented in FIG. 1 is also
configured to maintain a proper environment surrounding the
suspended structures (e.g., proof mass 14, fingers 24 and 26, and
tethers 30), by avoiding unnecessary variations in trench width. An
important example is the avoidance of intersecting trenches that
would create a localized wider gap prone to more rapid etching and
subsequent backside erosion, lateral erosion and vertical notches.
Accordingly, the only intersecting trenches are the finger and
tether trenches 28 and 32, which necessarily intersect along the
length of each tether 30 in a manner that minimizes the trench
width variation.
[0037] As noted above, the DRIE process of the present invention
also entails appropriate placement of the stiction bumps 34 to
avoid being eroded by the etching phenomenon associated with the
energetic, highly charged environment of the DRIE process. Because
the DRIE process more rapidly etches the proof mass fingers 24,
with the result that the mask 40 is undercut, any bumps formed on
the fingers 24 would also be rapidly etched and rendered
ineffective. As a solution, the present invention relocates the
interfinger stiction bumps 34 to the rim fingers 26 by
appropriately masking the rim fingers 26, as shown in FIG. 6.
[0038] The highly charged environment of the DRIE process has been
shown to increase the likelihood of undesirable translation and
stiction in the Z-direction of the device 10, possibly as a result
of the static charge build-up discussed above. Though the proof
mass 14 of the device 10 is particularly stiff in the Z-direction
because of the tether design, stiction of the proof mass 14 to the
floor 18 of the cavity 20 has been unexpectedly found to be a major
yield problem when etching is performed by DRIE. Accordingly, as a
direct result of implementing the DRIE process for a mass-produced
MEMS device, the present invention provides the stiction bumps 36
formed on the floor 18 of the cavity 20. According to the
invention, the bumps 36 are preferably placed directly beneath the
proof mass 14, as shown in FIG. 5. A difficulty with forming the
bumps 36 is the proper positioning and sizing of the bumps 36 as a
result of the highly-directional cavity etch and the DRIE etch. The
present invention has shown that the bumps 36 should be placed on
the cavity floor 18 away from the edges of the cavity 20 and the
trenches 17, 28 and 32 in order to prevent energetic etch species
reflection off of the bumps 36 toward the backsides of the
cantilevered fingers 24 and 26 and tethers 30, the result of which
would be undesirable notching of the structures.
[0039] Also a factor in location of the bumps 36 is the high
temperature bond oxidation process following the cavity etch, by
which the semiconductor layer 12b is bonded to the substrate 12a.
The bond process creates a vacuum within the initially sealed
cavity 20 as the die 12 returns to room temperature. As a result,
the semiconductor layer 12b is elastically pulled downward into the
cavity 20 until the cavity 20 is breached by one of the trenches
17, 28 or 32 during the DRIE etch; most preferably, the cavity 20
is breached first by the hub trench 17, which can be readily sized
to provide for controlled venting of the cavity 20 away from the
relatively fragile fingers 24 and 26 and tethers 30 without
adversely affecting the etching process and device performance. The
bumps 36 formed during the cavity etch are preferably placed away
from regions of maximum deflection of the semiconductor layer 12b
so as not to be a source of plastic deformation of the proof mass
14 later defined by etching the semiconductor layer 12b.
Accordingly, the stiction bumps 36 of this invention are located
uniformly around the inner and outer perimeters of the future proof
mass 14, but not beneath any trench or other opening formed through
the semiconductor layer 12b. The bumps 36 are also preferably sized
to prevent stiction of the proof mass 14 during severe Z-direction
translations, while not being so large as to contact the
semiconductor layer 12b when deflected as a result of the vacuum
within the cavity 20. For this reason, an optimal height for the
stiction bumps 36 is believed to be on the order of about
one-fourth to about three-fourths of the depth of the cavity 20,
which in practice is about eleven micrometers, though shallower or
deeper cavities could also been successfully used. Because wet
etches suitable for forming the cavity 20 are highly directional in
silicon, resulting in different etch rates along different
crystalline planes, it will be appreciated by those skilled in the
art that the rate at which the height of a stiction bump 36
decreases during the wet etch is a fairly complex function of the
etch rates of the exposed facets (silicon directions). Thus,
suitable modeling is preferably employed to obtain stiction bumps
36 having the prescribed height.
[0040] Those skilled in the art will appreciate that conventional
silicon processing techniques and materials can and would be
employed in the fabrication of a MEMS device, beyond those
discussed above. In addition, while a particular configuration is
shown for the proof mass 14, fingers 24 and 26 and tethers 30,
various modifications could be made by one skilled in the art. More
particularly, the present invention is applicable to essentially
any suspended or cantilevered structure that is DRIE etched over a
cavity. Finally, it is foreseeable that the present invention could
be utilized to encompass a multitude of applications through the
addition or substitution of other processing or sensing
technologies. Therefore, while the invention has been described in
terms of a preferred embodiment, other forms could be adopted by
one skilled in the art. Accordingly, the scope of the invention is
to be limited only by the following claims.
* * * * *